Add documentation on how to convert F3Net model (#4863)

* Add instruction for F3Net model pytorch->onnx conversion

* Fix style

docs/MO_DG/prepare_model/convert_model/onnx_specific/Convert_F3Net.md

@@ -0,0 +1,28 @@
+# Convert PyTorch* F3Net to the Intermediate Representation {#openvino_docs_MO_DG_prepare_model_convert_model_onnx_specific_Convert_F3Net}
+
+[F3Net](https://github.com/weijun88/F3Net): Fusion, Feedback and Focus for Salient Object Detection
+
+## Download and Convert the Model to ONNX*
+
+To download the pretrained model or train the model yourself, refer to the 
+[instruction](https://github.com/weijun88/F3Net/blob/master/README.md) in the F3Net model repository. Firstly, 
+convert the model to ONNX\* format. Create and run the script with the following content in the `src`
+directory of the model repository:
+```python
+import torch
+
+from dataset import Config
+from net import F3Net
+
+cfg = Config(mode='test', snapshot=<path_to_checkpoint_dir>)
+net = F3Net(cfg)
+image = torch.zeros([1, 3, 352, 352])
+torch.onnx.export(net, image, 'f3net.onnx', export_params=True, do_constant_folding=True, opset_version=11)
+```
+The script generates the ONNX\* model file f3net.onnx. The model conversion was tested with the repository hash commit `eecace3adf1e8946b571a4f4397681252f9dc1b8`.
+
+## Convert ONNX* F3Net Model to IR
+
+```sh
+./mo.py --input_model <MODEL_DIR>/f3net.onnx
+```

docs/doxygen/ie_docs.xml

@@ -55,6 +55,7 @@ limitations under the License.
                             <tab type="user" title="Converting DLRM ONNX* Model" url="@ref openvino_docs_MO_DG_prepare_model_convert_model_onnx_specific_Convert_DLRM"/>
                             <tab type="user" title="Convert PyTorch* QuartzNet Model" url="@ref openvino_docs_MO_DG_prepare_model_convert_model_onnx_specific_Convert_QuartzNet"/>
                             <tab type="user" title="Convert PyTorch* YOLACT Model" url="@ref openvino_docs_MO_DG_prepare_model_convert_model_onnx_specific_Convert_YOLACT"/>
+                            <tab type="user" title="Convert PyTorch* F3Net Model" url="@ref openvino_docs_MO_DG_prepare_model_convert_model_onnx_specific_Convert_F3Net"/>
                         </tab>
                         <tab type="user" title="Model Optimizations Techniques" url="@ref openvino_docs_MO_DG_prepare_model_Model_Optimization_Techniques"/>
                         <tab type="user" title="Cutting off Parts of a Model" url="@ref openvino_docs_MO_DG_prepare_model_convert_model_Cutting_Model"/>